xlviii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



to connect changes of fauna with any special gap in time accom- 

 panied by extreme unconformity ; for these changes may perhaps be 

 due to mere local variations of conditions encouraging for a time the 

 presence of special marine, estuarine, and freshwater or terrestrial 

 faunas or floras in certain areas. The strongest positive exception 

 to this seems to be in the rocks below and in those above the 

 Millstone-grit (where we have at least one unconformity in Dean 

 Forest) ; for the decidedly marine forms (Brachiopoda, &c.) of the 

 true Coal-measures in Coalbrook Dale and elsewhere are nearly all 

 distinct from those of the Carboniferous Limestone. The same kind 

 of break in organic succession is found in Scotland, where, however, 

 the coals in the Limestone-series point to a set of conditions closely 

 resembling those that prevailed during the formation of the ordinary 

 Coal-measures, and in that country no proof has yet been adduced of 

 any break in the stratification. 



Permian Strata. — When, however, we come to the Permian series, 

 the proofs of a remarkable physical break are unmistakeable. The 

 evidence of this in Britain is so well known that to bring it to 

 recollection I have only to state that the Rothliegende lies as it were 

 at hap-hazard on all our formations, from the Lower Silurian of 

 Malvern to the uppermost Coal-measures of the middle and north of 

 England. In part of the Rothliegende a few of the Plants of a very 

 meagre flora are common to the Coal-measures ; and when we come 

 to the marl-slate and limestone, all the Fish are distinct from those 

 of Carboniferous age. Of more than 170 genera and many more 

 than 1000 species of Carboniferous forms of marine life, only a few 

 survive to pass into the Permian series. Mr. Davidson is of opinion 

 that about half the Permian Brachiojooda are Carboniferous species, 

 — a much larger allowance than any one else has given. But this 

 would only give 10 or 11 species, and, adding other forms of shells 

 still doubtful, probably not 15 per cent, of the whole Carboniferous 

 fauna is common to the two epochs. As in former cases, I connect 

 this great break in the succession of species with a lapse of time in 

 our area, stratigraphically unrepresented, during which most of the 

 Carboniferous species died out or changed ; a few survived, pre- 

 serving their identity, and the others, according to Mr. Darwin's 

 views, are modified descendants of older forms. 



It is well known to some that I entertain peculiar opinions 

 respecting the glacial origin of those boulder-beds which here and 

 there form so large a part of the Rothliegende ; and I cannot help 

 connecting the smallness of the number of our Permian species, and 

 their dwarfed character, with a cold episode in this portion of the 

 geological record, analogous to that which seems to have produced a 

 dwindling of life in the northern hemisphere during the newer 

 Pliocene glacial epoch. 



After this comes the great break between the Palaeozoic and 

 Mesozoic strata, a part of the subject on which I will not at present 

 enter. 



General Considerations. — If we now review the stratigraphical and 



